s. sundar kumar iyer
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S. Sundar Kumar Iyer
Samtel Centre for Display Technologies
Organic Solar Cells
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Outline
Motivation ■ Solar cells■ Organic solar cells
Background■ Working of organic solar cell■ Fabrication steps
Research at IIT K■ Molecule, device, circuit and system level
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Clean Energy Supply Needed for Quality of Life
Fossil and nuclear fuels are costly■ If we include the environmental cost
The sun shines on everyone ■ Ideal for distributed power generation and remote locations
Tap solar energy directly■ Ideal for distributed power generation■ More environmentally friendly
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Annual Mean Global Irradiance
On a horizontal plane at the surface of the earth W m-2 averaged over 24 h
With 10% efficient solar cell area of solar cell needed in 2004India 60 km × 60 km (0.12% area) World need: 350 km × 350 km
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History
1839 Photovoltaic effect discovered by Edmond Becquerel
1954 First Silicon Solar Cell Bell Lab by Chapin, Fuller and Pearson
(
1970s Surge in research to harness solar energy
1986 Heterojunction Organic Solar Cell by Tang of Eastman Kodak
2007 Highest efficiency solar cells with ~40.7% in Spectrolab
A big surge in solar cells research & development is underway
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1mm
Efficiency ≈ 6 %
Chapin et al. 1954
The Birth of Silicon Photovoltaics
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Space Applications
www.spacetoday.org
marsrovers.nasa.gov
Photovoltaics are the mainstay
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Remote Locations
www.dacres.org
web.worldbank.org summitclimb.com
Photovoltaics are attractive
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Consumer Electronics
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Grid Supply
www.e2tac.org
www.sun-consult.de
Need to make photovoltaicsattractive in the marketplace
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Solar Energy Usage and Pricing
Solar markets
(average of last 5 years)
Solar Price/Competing
Energy source
Remote Industrial 17% 0.1-0.5 times
Remote Habitation 22% 0.2-0.8 times
Grid Connected 59% 2-5 times
Consumer Indoor 2% n/a
http://www.solarbuzz.com/StatsCosts.htm (2006 data; accessed 29.02.2008)
Solar Energy: 30 c (Rs. 12) per kWhNeed to lower cost to 10c (Rs.4) per kWh and below
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Electricity Generation Cost
Energy Source Cost
Combined cycle gas turbine 3 ¢ -5 ¢ (Rs.1.20-Rs.2.00)
Wind 4 ¢ -7 ¢ (Rs.1.60-Rs.2.80)
Biomass gasification 7 ¢ -9 ¢ (Rs.2.80-Rs.3.60)
Remote diesel generation 20 ¢ -40 ¢ (Rs.8.00-Rs.16.00)
Solar PV central station 20 ¢ -30 ¢ (Rs.8.00-Rs.12.00)
Solar PV Distributed 20 ¢ -50 ¢ (Rs.8.00-Rs.20.00)
http://www.solarbuzz.com/StatsCosts.htm (2006 data; accessed 29.02.2008)
13R.M. Margolis 2003
Solar Energy Production and Price
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Silicon Wafer40%
Cell Processing25%
Module35%
Data from A. Rohatgi
Cost Breakdown of Silicon Photovoltaics
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Lowering Cost of Solar Cells
Thin Film Solar Cells■ Multiple junction solar cells (a-Si:H, a-SiGe:H)■ CdTe based cells (CdTe, CdS)■ CuInSe2 (CIS) Ternary & Multinary compound solar cells
■ Multicrystalline/Microcrystalline silicon solar cells■ Thin film GaAs solar cells■ Organic solar cells
S. Deb 2004
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Organics PhotovoltaicZweibel et al. 2004
Spectrolab 40.7%
Efficiency of PV for Different Materials
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Why Organic Solar Cells?
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Printing ■ Screen Pringing■ Stamping
Spraying Spin Coating Vaporisation
High-Throughput and Low-Cost Processing
Step 1 Step 3Step 2
Deposition Patterning Packaging
Raw MaterialsPlastic RollsRaw MaterialsPlastic Rolls
Finished Goods(Solar Cell)Finished Goods(Solar Cell)
A simplified overview
www.rolltronics.com
Roll-To-Roll
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Flexible Solar Cells
Flexible Surfaces
Conformal Surfaces
Prof. Kippelen’s Group; Georgia Tech
Example show is a CIGS solar Cells
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Eco-Friendly Technology
Appropriate Process
Biodegradable Molecule
Modifiedchromophore of Green Fluorescent Protein Molecule
Anode contacts
Cathode contacts
Solar cell device using the molecule
Ink Reservoir
Chamber
Nozzle
Piezoelectric crystal
PulsedSignal
Epson
Example: Ink-jet technology uses material only where needed
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Background
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Efficiency of a Solar Cell
Fill Factor FF is the ratio of
area of maximum rectangle
fitted in the 4th quadrant I-V
and the product of VOC and ISC
Maximum Power Output
Pmax = VOC × ISC × FF
Efficiency
=
p n
V
I
I (m
A)
V (V)
VOC
ISC
I
V
Max Power
Rectangle
Dark
Light
S.M.Sze 1991
Pmax
Incident Optical Power
RL
h
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Classic p-n Junction Photovoltaic Cell
• Incident photon immediately forms mobile electrons and holes
e-
h+Ev
Ec
hEg= Ec - Ev
bi
n-type p-type
Ebuilt-in
Efn
Efp
h
Inorganic Semiconductor
Ef
-ve +ve
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hν
h+
Exciton
e-
h+
e-
Hole Transport Layer
Electron Transport Layer
CathodeAnode
Photon Absorption Exciton FormationExciton DiffusionExciton DissociationCharge Transport & Collection
Photon Absorption Exciton Formation
Exciton Dissociation
Exciton Diffusion
Charge Transport & Collection
e-
EHP Formation
Organic Solar Cells Operation
A Heterojunction Organic Solar Cell Structure
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Photovoltaic Process In Organic Solar Cells
LightReflected
Away
PhotonsNot
Absorbed
Couplingof sunlight
into solar cell
Absorptionof
incidentphotons
Creationof ‘free’ charges
Separationof chargesby built-inE field
ChargesRecombine
ChargesRecombine
Collectionof charges
atelectrodes
Creationof
excitons
ExcitonsRecombine
Su
nli
gh
t
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ITO Patterning
PEDOT:PSS Coating
Active Layer Deposition
Metal Deposition
Device Fabrication
PEDOT:PSSActive Layer
Ca
Ca
ITO
Al
Al
Contacts+ + + +
-
-
Transparent Glass Substrate
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www.sciencemag.org SCIENCE VOL 317 13 JULY 2007 pp. 223-225
Tandem Cell: Jsc = 7.8 mA cm-2, Voc = 1.24 V, FF = 0.67 and = 6.5%
Highest Efficiency Reported OSC Till Date
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Organic Solar Cell Work at IIT K
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The Team
Prof. Satyendra Kumar (Physics)
Dr. Ashish Garg (MME)
Prof. Baquer Mazhari (EE)
Prof. R. Gurunath (Chemistry)
Dr. S.P. Das (EE)
Dr. P.S. Sensarma (EE)
Dr. R.S. Anand (EE)
Dr. Vibha Tripathi (EE)
Prof. Y.N. Mohapatra, Prof. Deepak Gupta, Prof. Monica Katiyar, Dr. Siddhartha Panda, Dr. Narain, …
S. Sundar Kumar Iyer
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The Processing Laboratory
ISO 6, 220 m2
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Characterisation Facilities
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Three Pronged Approach
Increasing efficiency of device■ Physics and circuit model of organic solar cells■ Choice of Material■ Structure – Blend, Bilayer, Tandem …■ Process Optimisation
Reliability and Stability■ Choice of Material■ Mechanism of Degradation■ Encapsulation Techniques
New & emerging technology issues■ Novel methods of fabrication■ System level issues
CuPC
PCBM
R = Hexyl group
P3HT
Stable MoleculeFrom P3HT FamilyAVPV Indium Tin
Oxide LinesPEDOT:PSSActive Layer
Contact to Cathode
Encapsulation
ITOContact to AnodeGlass
Printed Ink
EngravedCells
Substrate
+
-
V
I
RSH
RS
IL
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Organic Solar Cell Model
IL is a function of voltage
Exciton generation IP is a constant
B. Mazhari 2006
IP
Rshunt, int.
D1
D2
Vint
Rs, int.
Ddark
+
-
V
I
RSH
RS
New Model
+
-
V
I
RSH
RS
IL
Traditional Model
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Optical Efficiency O
Optical losses maybe due to
■ Reflection at the surface
■ Unabsorbed light leaking out
Solutions
■ Anti Reflection Coating (ARC)
■ Texturing the top surface
■ Concentrators
■ Thickness of layersDevice
Back electrode
O = 1-R where
R =(n1-n0)2 + 2
(n1+n0) + 2
ni : refractive index of medium i: attenuation coefficient in device
n0=1 for air
n1,
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Light Trapping by TiO2 Nanoparticles
Device
Back electrode
300 500 700 900 (nm)
100
80
60
40
20
0
Ref
lect
ance
(%
) P3HT:PCBM + TiO2
P3HT:PCBM
Jyoti Singh 2008
TiO2 particle is dispersed in the P3HT:PCBM blend
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Cathode Variation
Illumination: AM1.5D 100 mW cm-2
Nitin Sahai 2008
Glass
Active Area
AlCa
ITO
Cu
rren
t D
ensi
ty (
mA
cm
-2)
Voltage (V)
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Effect of Post Process Anneal
P3HT: PCBM BlendHeterostructure
Vinod Pagare 2007
Aluminium Cathode
Polymer Blend PEDOT:PSS
ITO Glass
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Degradation Models
Munish Jassi 2006
Degradation under Electrical & Optical Stress
• Statistically arrive at parameters that matter most• Identify the physics of degradation• Use learning to increase device lifetime
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Summary
Organic solar cells offers unique opportunities in future■ Low-cost high volume production■ Distributed production■ Environmentally benign devices
Work at IIT Kanpur■ Molecule and material level■ Process■ Device level■ Circuit level■ System level
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Let us make Organic Solar Cells Happen!
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